1. Field of the Invention
The present invention relates to a projection display apparatus using a light valve.
2. Description of the Background Art
In general, a conventional projection display apparatus mainly includes a light valve, a light source for emitting light to the light valve, a guiding optical system for guiding the light from the light source to the light valve, an integrator lens disposed on the optical path of the light from the light source to the light valve, for uniformizing the illuminance distribution of the light from the light source, and a projection lens for projecting images produced by the light valve onto a given screen.
In such a conventional projection display apparatus, various optical elements constituting the optical systems, such as the guiding optical system, the projection lens, etc., cause light leakage and stray light, and so the brightness/darkness in the image projected on the screen is not clear, and sufficient contrast cannot be obtained. The term “contrast” herein means the degree of brightness/darkness of an image.
In particular, in projection display apparatuses using liquid-crystal light valves, the contrast of images depend also on the performance of the liquid-crystal light valve, and there is a limit in improving the image contrast by image signal processing.
A method to solve this problem is to control the amount of light that illuminates the light valve according to the image signal, so as to improve the image contrast.
Then, a simplest method to control the amount of light that illuminates the light valve is to control the optical output of the light source used in the projection display apparatus to control the amount of light that illuminates the light valve. However, controlling the optical output of the light source is extremely difficult. Furthermore, in such methods of controlling the optical output of the light source, the brightness is varied in such short periods as can be perceived by human eyes, which causes flicker in the image projected on the screen.
As to techniques to solve this problem, a projection display apparatus is disclosed in which a light control means composed of a segmental liquid-crystal device, whose transmissivity can be controlled in each segment, is provided on the entrance side, or the exit side, or between, two lens arrays forming an integrator lens, and the transmissivity of each segment of the light control means is controlled on the basis of the image signal, whereby the amount of light illuminating the light valve can be controlled without controlling the optical output of the light source (i.e., without causing flicker in the image projected on the screen: for example, see Japanese Patent Application Laid-Open No. 2003-131322, Paragraph (0024), which is hereinafter referred to as Conventional Art 1).
A projection display apparatus according to another technique uses an amount-of-light control device in place of the light control means of Conventional Art 1, in which the amount-of-light control device uses light-blocking plates for blocking the optical path from above and below, or from right and left, or a diaphragm mechanism for blocking the optical path, whereby the amount of light illuminating the light valve can be controlled without controlling the optical output of the light source (for example, see Japanese Patent Application Laid-Open No. 2003-241311, Paragraph (0034) and FIG. 2, which is hereinafter referred to as Conventional Art 2).
A projection display apparatus according to still another technique uses light-blocking plates in place of the light control means of Conventional Art 1, in which the light-blocking plates are provided like hinged double doors on both sides of the optical path between two lens arrays, and are turned to control the extent of blocking of the optical path, whereby the amount of light illuminating the light valve can be controlled without controlling the optical output of the light source (for example, see Japanese Patent Application Laid-Open No. 2005-31103, Paragraphs (0073), (0074) and FIG. 15, which is hereinafter referred to as Conventional Art 3).
Also, a projection display apparatus according to still another technique uses light-blocking plates in place of the light control means of Conventional Art 1, in which the light-blocking plates are provided on the left and right sides and the upper and lower sides of the optical path between two lens arrays, and are slid approximately in parallel with the main surface of the lens array to block the optical path, whereby the amount of light illuminating the light valve can be controlled without controlling the optical output of the light source (for example, see Japanese Patent Application Laid-Open No. 2005-31103, Paragraph (0069) and FIG. 13, which is hereinafter referred to as Conventional Art 4).
However, in Conventional Art 1, the light control means, which is composed of liquid crystal, suffers from reduced lifetime due to the heat generated by the light from the light source (particularly, ultraviolet rays affect the lifetime of the light control means). In particular, when the light control means is disposed on the entrance side of, or between, the two lens arrays, a larger amount of heat is accumulated in the light control means and so the lifetime of the light control means is more likely to be reduced, because, in these positions, the light flux is less attenuated by influences of optical elements than in other positions, and therefore the light flux from the light source is more intensive in these positions.
Also, in Conventional Art 4 using light-blocking plates, the light-blocking plates have their forward ends formed flat, and so they cannot smoothly control the amount of light directed to the light valve when they are moved continuously. Thus, the amount of light cannot be smoothly controlled according to the image signal, to improve the contrast.
Conventional Art 2 using a diaphragm mechanism involves a complicated structure.
In Conventional Art 3 using light-blocking plates turned like hinged double doors, when the light-blocking plates are turned and their forward ends pass near the lens array closer to the light source, the shape of the forward ends may be imaged to the image projected on the screen, which will then appear as illuminance non-uniformity.